1. Field of the Invention
The present invention relates to a second harmonic oscillator that operates mainly in a microwave or millimeter wave domain.
2. Description of the Related Art
Along with the widespread of consumer radars such as an on-vehicle radar and a cellular phone, a demand for downsizing an oscillator whose output frequency exceeds 1 GHz and enhancing the performance of the oscillator has been increased. The “oscillator” refers to a circuit that oscillates an electric signal in the interior of the circuit and transmits the electric signal to the external.
In a fundamental harmonic oscillator, in a case where an electric signal having a desired frequency is generated with excellent characteristics (high output, low phase noise), it is necessary for a transistor to have a sufficient gain at the desired frequency. However, the gain of the transistor generally decreases as the frequency becomes higher. Therefore, the harmonic oscillator is generally used. The “harmonic oscillator” refers to an oscillator that oscillates an electric signal that is a factor of an integer of the desired frequency, and takes out a harmonic signal from an output terminal. In the harmonic oscillator, a demand for the high frequency characteristics with respect to the transistor is not severe as compared with a fundamental harmonic oscillator. Thus, an oscillator having excellent characteristics can be obtained.
FIG. 5 is a diagram showing the configuration of a related art second harmonic oscillator. In FIG. 5, a matching circuit 4 reflects a fundamental harmonic signal from a transistor 1 toward an output terminal 5, and takes out a second harmonic signal to the external. In the case of the second harmonic oscillator, there is a circuit using a leading end open stab having a line length corresponding to one-fourth of the wavelength of the fundamental harmonic signal as an example of the matching circuit 4 (for example, refer to “A low Phase Noise 19 GHz-band VCO using Two Different Frequency Resonators,” IEEE MTT-S Int. Microwave Symp. Digest, pp. 2189-2191, 2003).
There is a push-push oscillator (for example, “A monolithic integrated 150 GHz SiGe HBT Push-Push VCO with simultaneous differential V-band output,” IEEE MTT-S International. Microwave Symposium. Digest, pp. 877-880, 2003) as an oscillator that takes out a second harmonic signal besides the second harmonic oscillator using the above matching circuit 4.
Also, in a Colpitts harmonic oscillator of an emitter follower configuration having a low frequency (1 GHz or lower), there has been proposed installation of a circuit that comes to an open state at a fundamental frequency and a short-circuit state at a harmonic frequency to the emitter of the oscillator transistor as an approach to improve the harmonic output electric power (for example, refer to JP 08-23229 A). With the above configuration, there has been reported that the circuit blocks the feedback of the harmonic signal of the Colpitts harmonic oscillator, thereby making it possible to improve the harmonic output electric power.
The suppression of phase noise is required in an oscillator. In other words, it is required that the frequency spectrum of the output signal be sharp in a delta function fashion. In addition, in recent years, it is also required that the output power be high in addition to the low phase noise. This is because the number of stages of multistage amplifiers that are used to amplify the output power of the oscillator can be reduced by improving the output power of the oscillator that is used in a consumer radar or a cellular phone, and the low cost and the low power consumption are attained for the entire system thereby.
If there is no upper limit to the physical size of the oscillator, it is possible to produce an oscillator that is high in output and low in phase noise by an aid of a resonator that is large in physical size. Also, an approach of combining output powers of plural oscillators can be taken.
However, in many actual cases, the upper limit of the physical size of the oscillator is fixed to some extent. In the case where the physical size of the oscillator is fixed, most approaches to improve the output power deteriorate the phase noise.